Demand for a mobile computer, such as a personal computer and a laptop computer, has increased rapidly over the years. Consequently, the demand for a small size magnetic disk drive has sharply increased as a memory device for such a mobile computer. Moreover, increases in the recording density of memory devices have caused problems which relate to the positioning accuracy of magnetic heads for disk drives. Further, request for high speed access and low power consumption has caused an intensive demand for highly efficient access motors within the drives.
A flat coil type voice coil motor (VCM) is used generally as the access motor for small size magnetic disk drives. The VCM is structured by vertically stacking an upper and lower yoke of an almost trapezoidal shape, attaching a magnet to at least one piece of the yoke to form a magnetic circuit, and arranging a flat coil of almost the trapezoidal shape between the upper and lower yokes. The coil is provided at one end of a carriage which rotatably holds a plurality of magnetic head arms at the other end.
For practical purposes, a structure of such magnetic circuits is disclosed in Japanese Laid-Open Patent Application Nos. 4-302874,6-251515 and 7-78427. Japanese Laid-Open Patent Application No. 6-251515, for example, discloses a magnetic circuit where one end of the lower yoke is fixed by a first screw and the other end is fixed by a second screw threaded through the upper yoke and a spacer, to the bottom wall of the drive base. Because the yoke must be positioned accurately, it is crucial that the screw holes of the upper yoke, the lower yoke, and the drive base be positioned accurately. As a result, the manufacturing process can be both difficult and expensive.
Moreover, in the case of mounting the upper yoke using a screw and pin, a height space as much as the head of the screw or pin is necessary between the upper surface of the upper yoke and the drive cover. Therefore, the total size of the magnetic disk drive must be increased, or a design change of the magnetic circuit must compensate for the additional space required by the screw and pin. Accordingly, a part of the yoke must be formed thinner, or the height of the magnetic circuit must be changed, resulting in lower performance characteristics from the motor. Thus, there is a need for a yoke which incorporates a simplified structure having fewer parts and allowing for a decrease in the total size of the disk drive, without affecting the performance of the motor.
Further, Japanese Laid-Open Patent Application No. 7-78427 discloses a base with a circuit having a structure of rectangular holes provided to the lower yoke and rectangular projections provided to the upper yoke. These holes and projections engage each other to position the upper yoke on the lower yoke, without utilizing holes or projection in the base. Because the upper yoke is positioned solely with the lower yoke, the configuration requires high processing accuracy and positional accuracy of the holes of the lower yoke and the projections of the upper yoke, and high fitting accuracy of the drive base and lower yoke.
This configuration mandates clamping a yoke stopper to the side of the upper yoke, to improve the vertical and horizontal positioning accuracies of the upper yoke and lower yoke. Additionally, the structure of the magnetic circuit requires that a damping material be provided between the upper and lower yokes. Therefore, as in the related art above, there exists an increase of cost due to an increase in the number of parts and manufacturing steps necessary to maintain acceptable magnetic circuit characteristics and stable performance. Namely, considerable assembly accuracy and an increased number of parts are required to realize accurate positioning in the vertical and horizontal directions of the upper and lower yokes.
Realizing high speed access of the magnetic heads requires high precision relative to the positional relationship between the magnetic circuit and flat coil. In other words, efficiency of the voice coil motor can be lowered depending on the positioning accuracy in the vertical and horizontal directions of the upper and lower yokes. For example, if the upper and lower yokes are arranged with horizontal deviation, the magnetic effect at the end part of the yokes is lowered, a seeking time for positioning the magnetic disk to the inner most or outer most track of the disk is raised, and the magnetic disk cannot be positioned to the target track due to insufficient power. Thus, there is a need for highly accurate vertical and horizontal positioning of the upper and lower yokes.
Accordingly, one object of this invention is to provide new and improved yokes for memory devices.
Another object is to provide new and improved yokes of reduced size to be used in small magnetic disk drives.
Yet another object of the present invention is to provide new and improved yokes which incorporate a simplified structure consisting of fewer parts and allow a decrease in the total size of disk drives.
Still another object of the present invention is to provide new and improved yokes for voice coil motors in memory devices which assure a required assembly accuracy of the magnetic circuit to enable high speed access of heads.